Update gorilla/mux and gorilla/handlers to latest stable releases

Hayley Swimelarrequested to merge
update-mux into master

This MR bumps mux from mux v1.7.3 to v1.8.0 https://github.com/gorilla/mux/releases and the handlers from a pre v1.0.0 release from 2015 to v1.5.1 https://github.com/gorilla/handlers/releases

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